1. Field
The present disclosure relates to a MIMO (multi-user MIMO: hereinafter referred to as “MU-MIMO”) communication method that uses MIMO (multiple-input and multiple-output communication), and to a transmitting device and a receiving device.
2. Description of the Related Art
There have been an increasing number of situations where mobile communication terminals and many other kinds of devices perform communication via radio networks. This is expected to be more noticeable in a situation where LTE (Long Term Evolution), which is a communication standard that has started to be put into practical use, and its extended versions, or LTE-Advance and LTE-Evolution, are put in practical use. This situation is a worldwide trend, and analysis performed by 3GPP (3rd Generation Partnership Project) predicts a case where, for example, the number of smartphones will sharply increase, resulting in congestion of radio traffic. In particular, for communication terminals such as smartphones, there is predicted a case where communication between many devices is out of human control and congestion of radio traffic occurs.
As one of the countermeasures against the above-described situations, LTE currently adopts a modulation scheme based on OFDM (Orthogonal Frequency Division Multiplexing) to achieve an increase in communication capacity. That is, in LTE, currently, a multi-carrier technology is used to increase frequency utilization efficiency. In addition, a system with a maximum bandwidth of 100 MHz will be proposed in future release of the LTE standard.
However, serious congestion of traffic is expected in the near future even with the use of such a method. That is, analysis performed by 3GPP predicts that such congestion might not be sufficiently accommodated by a conventional method in which the communication speed is increased and the number of frequency bands used is increased to increase communication channel capacity.
As one of the countermeasures against this, MU-MIMO (multi-user MIMO) has been proposed. MU-MIMO is a communication scheme defined in Transmission Mode 5 in 3GPP standardization of Rel.8. For example, for uplink communication from a plurality of (N) terminals in a single cell to a base station by applying MU-MIMO to LTE, the base station prepares N receive antennas. The individual terminals perform communication of completely different contents during the same time period and at the same frequency, and the base station receives the contents using the N receive antennas. The base station utilizes orthogonality (correlation) between channels to identify the terminals from which signals have been received.
There also has been studied an extended version in which MIMO of a plurality of terminals and a base station controller (BSC) that collectively manages a plurality of cells is constructed in units of BSCs.
By applying MU-MIMO, there is a beneficial effect of potentially increasing the utilization efficiency of frequency resources and increasing the communication channel capacity, and MU-MIMO is being studied as a technology to address a future increase in communication traffic to some extent.
In MIMO, channels are expressed in terms of a matrix based on a combination of antennas on the transmitter side and antennas on the receiver side. For example, in 3GPP, a configuration with up to 16 transmit antennas and 16 receive antennas is proposed. In this case, a 16×16 matrix (hereinafter referred to as an “H-matrix”) is obtained. That is, the base station needs to separate and receive communication from up to 16 terminals using the inverse matrix of the H-matrix. Because of the difficulty of a technique for accurately separating signals of 16 channels using the inverse matrix of such a 16×16 H-matrix, actually, no more than 4×4 or 8×8 MIMO will possibly be put into practical use. Such MU-MIMO is being prepared for practical use by communication providers through experiments in their research institutions.
MU-MIMO requires the inverse matrix of the H-matrix to be determined on the receiver side (base station side). Generally, the elements of the H-matrix (that is, instantaneous transfer functions for a channel) are determined on the terminal side using a reference signal or pilot signal (RS) transmitted from the base station, and the terminal feeds back the results to the base station. The base station constructs an H-matrix by using all the results, and determines the inverse matrix thereof. After communication is started, at reception, signals from all the terminals are received and then the receive signals from the respective terminals are separated using the inverse matrix. For utilization of the inverse matrix, zero forcing, the MMSE method, or the like is used.
Meanwhile, some terminals move at high speed. In addition, in a case where a terminal is used in an adverse environment such as a metropolitan area, the instantaneous transfer functions for a channel may constantly vary largely due to large amounts of fading and shadowing. Therefore, there is a need to update the H-matrix at certain short time intervals. That is, the base station needs to continue to frequently compute and update the inverse matrix. The computation of an inverse matrix requires a greater number of computation processes as the order of the matrix increases.
In the case of an Nth order square matrix, computation needs to be performed (N3×N!) times to determine the inverse matrix, and LU decomposition or the like is usually used for the fourth or higher orders, while high-speed computation requires time and a large amount of power consumption. Additionally, the inverse matrix does not always exist. That is, it does not exist in a case where the H-matrix is not regular. The higher the order, the more the possibility of the inverse matrix not being determined. If it is not possible to determine the inverse matrix, the orthogonality of channels collapses, resulting in MIMO not being established.
Accordingly, the work of determining the inverse matrix imposes a significantly large load on an MU-MIMO base station, leading to a delay of the processing time and an increase in power consumption. Furthermore, because MIMO is not established, there is a problem in that the problem of traffic congestion is not substantially resolved.
Furthermore, to increase reception quality through diversity at the base station, the number of receive antennas or receiving units required is doubled, and MU-MIMO becomes more difficult to implement as the order of MU-MIMO increases.
The inventor has recognized the difficulty in implementing MU-MIMO.